1. Field of the Invention
The present invention relates to an injection compression molding method of a molded article having a thick rib portion, and more particularly to an injection compressing molding method using both clamping force and gas pressure.
2. Description of the Related Art
Conventionally, there has been widely known a gas injection molding method in which gas is injected into a thick portion of the molded article to make it hollow in order to improve sinkages in the molded article.
In such a gas injection molding method, it is necessary to charge molten resin into a mold cavity at an extremely low pressure in order to inject gas into the molten resin. As a result, the transferability of the mold is deteriorated, and the appearance of the molded article is apt to be spoiled, so that fabricating such as painting or the like is required as occasion demands. In addition, is has been found that technical investigations including many things such as the arrangement of the thick portion, the size, the arrangement of the gate, and so on, are necessary for making a desired thick portion hollow. In addition, the gas pressure to be used is generally high, and consumption of the gas is also high, thereby causing increase in production cost. Further, when treatment such as plating, etc., is required, a plating solution is apt to invade the hollow portion from the hole of the gas injection port to generate a defect product. Therefore, troublesomely, a pre-process for sealing the hole of the gas injection port is required.
As an injection molding method for solving the problems with the aforementioned gas injection molding method, there is an injection compression molding method using both clamping force and gas pressure. For example, a method in which molten resin is injected in a state where a mold has not been closed perfectly yet, and gas is injected between the resin of a thick portion and the mold after the molten resin reaches the thick portion by clamping compression of the mold.
Such an injection compression molding method using both clamping force and gas pressure is regarded as effective on molding a molded article having a thick portion locally, such as a boss, etc. That is, sinkages are apt to be produced in the thick portion and spoil its appearance when such a molded article is molded by a usual injection molding method. On the other hand, in the above-mentioned injection compression molding method using both the clamping force and gas pressures, there is not such a problem and sinkages are prevented from being produced, and a molded article with a superior appearance can be obtained easily.
However, the above-mentioned conventional injection compression molding method using both the clamping force and gas pressures suffers from the following problems.
When gas is injected while clamping pressure is maintained after completion of mold clamping, it is indeed possible to make the gas pressure act directly on a locally thick portion such as a boss, etc. easily, but it is difficult to make the gas pressure act sufficiently all over a continuously thick portion such as a rib, etc., because of an increase in the pressure of molten resin in the mold caused by the clamping pressure being maintained. Therefore, when a continuously thick portion such as a rib, etc. is formed, it is necessary to increase the gas pressure or provide a number of gas injection ports. As a result, the design of the thick portion is inevitably limited for the sake of gas injection, and the level of sinkages in the obtained molded article cannot be satisfactory.